The bug

If you place a large number of copper blocks in the world it causes the game to start lagging and when you attempt to save the world it will take forever. I placed a few cubes of copper in a test world and when I tried to save the world it took 5 minutes and 30 seconds to save.

This is probably due to copper blocks each scheduling a tile tick that lasts at least 1200000 ticks flooding the tile tick list and causing a lot of issues.